Some conventional auto focus apparatuses condense reference light emitted from the light source for auto focus onto a sample, receives the reference light reflected from a sample using a photodetector, and positions the focal point of an objective lens based on this light receiving intensity (e.g. see Japanese Patent Application Laid-Open No. 2002-328297). In such an auto focus apparatus, a plate spring actuator mirror, which changes the optical path of the reference light by rotating the reflecting surface, is disposed, so that the reference light can be moved to a position different from the observation center (optical axis) of the objective lens by rotating the reflecting surface of the plate spring actuator mirror. Therefore even if there is a step difference in an observation field of the objective lens on the sample, the focal point of the objective lens can be stably positioned by moving the image forming position of the reference light.